1. Field of the Invention
The present invention relates generally to fifth wheel coupling devices and, more particularly, to a fifth wheel coupling device having a rotatable bearing plate.
2. Description of the Prior Art
Conventional fifth wheel coupling devices generally comprise a wheel plate mounted on the bed of a tractor or towing vehicle and defining a slot for receiving a kingpin extending downwardly from a support plate mounted to a trailer. After insertion, the kingpin is releasably retained in the slot by a suitable locking mechanism.
During coupling operations and under running conditions, the wheel plate and the kingpin support plate provide the sole bearing contact between the tractor and the trailer. The bearing surface of the wheel plate supports the weight of the trailer resting thereagainst it and acts as a surface against which the bearing surface of the kingpin plate rotates during towing operations. Although the so formed load bearing surfaces are relatively large, it has been found that axial friction during coupling, and oscillation and rotation during running result in premature and uneven wear of both load bearing surfaces. This can lead to many problems including, increased tire wear, dangerous steering characteristics as well as structural problems.
The normal practice is to provide a lubricant, such as grease, between these bearing surfaces. However, it has been found that even with lubrication there is still premature wear of the load bearing surfaces. Furthermore, the utilization of grease favors the accumulation of dirt between the top bearing surface of the fifth wheel plate and the opposing undersurface of the kingpin plate. This again results in excessive wear of the opposing bearing surfaces.
Moreover, rocking and turning action between the trailer and the fifth wheel squeezes the grease off the fifth wheel bearing surface and renders the steering progressively more difficult as the friction between the fifth wheel and the trailer increases.
To overcome the above-described problems, it has been proposed to provide a special mounting structure for rotatably mounting the kingpin plate to the chassis of the trailer. However, such structures have not gained overall acceptance since they are relatively complex, expensive and difficult to install.
Furthermore, it has been found that mounting rotatable kingpin plates on trailers is not an economical viable solution in that there are more trailers than there are tractors.
It is therefore an aim of the present invention to provide a new fifth wheel coupling device wherein friction between bottom and top bearing surfaces of the device is reduced.
It is also an aim of the present invention to provide a new fifth wheel having a rotatable top bearing surface.
It is a further aim of the present invention to avoid the necessity of applying a lubricant between a fifth wheel plate and a kingpin plate.
It is a still further aim of the present invention to minimize wear of a fifth wheel bearing surface.
It is a still further aim of the present invention to reduce the friction between a kingpin and a fifth wheel jaw of a fifth wheel.
It is still a further aim of the present invention to avoid the necessity of externally applying grease or a similar lubricant on a fifth wheel jaw.
It is still a further aim of the present invention, to provide a new fifth wheel having a ramp adapted to support a trailer during coupling and uncoupling operations.
Therefore, in accordance with the present invention, there is provided a fifth wheel for coupling a tractor to a trailer carrying a kingpin. The fifth wheel comprises a mounting structure adapted to be mounted to the tractor, a fifth wheel plate mounted to the mounting structure and having a top bearing surface rotatable relative to the mounting structure about an axis normal to the top bearing surface.
In accordance with a further general aspect of the present invention, there is provided a fifth wheel adapted to be mounted on a tractor for releasable engagement with a kingpin carried by a trailer, comprising a fifth wheel plate having a top bearing surface and defining a fifth wheel opening adapted to captively receive the kingpin, and a ramp adapted to support the trailer during coupling and uncoupling operations, wherein the fifth wheel plate is movable relative to the ramp between a lowered position in which the top bearing surface is located at a substantially lower elevation than the ramp so that the trailer be supported by the ramp and a raised position in which the top bearing surface is located at a higher elevation than the ramp to support the trailer when the kingpin is retained captive in the fifth wheel opening.
In accordance with a further general aspect of the present invention, there is provided a fifth wheel jaw for releasably retaining a kingpin in a fifth wheel opening of a fifth wheel plate. The fifth wheel jaw comprises at least one jaw member displaceable between open and closed positions, at least one roller rotatably mounted to the jaw member and adapted to engage the kingpin, when the jaw member is displaced to its closed position, so as to provide a low friction contact between the fifth wheel jaw and the kingpin.